Previewing 2019: the diverging diamond interchange

Visual concept for McDonough diverging diamond interchange (Georgia DOT photo)

Moving Henry Forward is previewing what we can expect in 2019. A major project that will enter construction is the diverging diamond interchange at I-75 and state route 20 / exit 218 in McDonough.

Construction is forecast to let out for bids in September 2019, though Georgia DOT staff have expressed the project could be ready to start as early as next summer. Construction is expected to last an estimated 10 – 12 months.

What is the DDI?

The diverging diamond interchange is an innovative roadway design that improves traffic operations by 30 – 40% while also reducing crash rates. The design originated in France during the 1970s and was first built in the United States in 2009.

The design utilizes a crossover, where vehicles travel on the left-hand side of the roadway within the interchange. As drivers approach an interstate exit, they crossover to the left-hand side at the first traffic signal within the exit. After traveling over the bridge, they cross back to the right-hand side in the second traffic signal. This allows left-hand turning vehicles to enter or exit the interstate without the need to turn in front of opposing traffic. Additional road signage, lane markings, and concrete medians help motorists navigate the interchange. Most drivers grow accustomed to the design after only a few trips through the exit.

A visualization is available from North Carolina DOT; I recommend this video for anyone wanting a visual aid to help understand the roadway design.

Where are DDIs in Georgia?

There are six diverging diamond interchanges open across Georgia. Most are located on the north side of Atlanta with one in Savannah.

  • I-285 at Ashford Dunwoody Road (DeKalb County), opened in 2012 as Georgia’s first DDI
  • I-85 at SR 140 / Jimmy Carter Blvd (Gwinnett County)
  • I-85 at Pleasant Hill Road (Gwinnett County)
  • I-75 at Windy Hill Road (Cobb County)
  • I-75 at Wade Green Road (Cobb County)
  • I-95 at SR 21 (Chatham County)

A DDI is under construction on I-285 at Camp Creek Parkway in East Point. With a completion date of November 2019, the location will become the first DDI in south metro Atlanta. Others are planned over the next few years in Augusta, Columbus, Savannah, and throughout metropolitan Atlanta.

How much will the project cost?

Construction is estimated at $4.7 million. This is a significant reduction over traditional interchange expansion projects because a DDI utilizes the existing bridge structure. By avoiding costly reconstruction of the bridge, the project saves money and can be completed sooner. For comparison purposes, the 2015 project to expand Jodeco Road incurred construction costs of $17.6 million. Interchange projects under construction at I-285 & Flat Shoals Road (DeKalb County) and I-285 & South Atlanta Road (Cobb County) have construction costs close to $50 million.

Once completed in 2020, the DDI will reduce congestion along state route 20 and help residents travel to commercial destinations around South Point. Other diverging diamonds should be considered in the future to help Henry County traffic.

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2 thoughts on “Previewing 2019: the diverging diamond interchange

  1. Well this is really gonna help the 75 south bound traffic. Seems like the express lane was a complete waste as I sit everyday in 40 minutes of traffic from morrow to exit 218.

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    1. The diverging diamond interchange reduces delays on the off ramp, thereby improving safety along the interstate main line. Often, vehicles waiting to exit at highway 20 back up into the regular lanes creating a major hazard. The DDI works to more efficiently move those waiting vehicles off the exit ramp and into the regular flow of highway 20.

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